Many applications rely on the ability to detect light signals traveling in a waveguide. Light propagates in straight line paths known as rays. Rays may be refracted, reflected, and scattered at material interfaces. In a dielectric waveguide, a high index core region may be surrounded by a lower index cladding layer. Rays may be confined in the high index core region by internal reflection at the core/cladding interface. The reflected rays may interfere with each other to form electromagnetic field patterns within the waveguide.
In a waveguide, light may have only certain allowed states called “modes.” “Modes” of a waveguide refer to field patterns that propagate in the core region without dispersion, i.e., changing shape. The waveguide could be “single mode” if it only supports one mode. “Multimode” waveguides support many modes. An analogy of a mode may be thought of as a probability function, where the mode is similar to electron shells in atoms. An electron is confined in a shell, just as a photon is found in its mode. By changing the shape of the waveguide, the waveguide mode can no longer be supported, and light may be expelled, directed, steered or “forced” out of the waveguide and into the higher index detector.
A waveguide may guide light to a photodetector. To electrically detect light in a waveguide, the photodetector may absorb radiation, collect photogenerated charge and produce an electric current. Currently, evanescently coupled waveguide photodetectors or partially evanescently coupled/butt coupled photodetectors are used to detect light in a waveguide. Typically, with evanescent coupled photodetectors, the coupling is relatively weak and devices 20-100 microns long are needed to capture the light efficiently. This leads to higher dark current and detector capacitance which could limit the speed of the device.